linux-kernel - Re: [RFC PATCH 00/31] netfs: [WIP] Allow the use of MSG_SPLICE

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Message-ID: <q4ngijgig45tompgxwc7eu2odtjp65lby2lx6bpbvu3sw2inlm@mfpunzg4uaur>
Date: Mon, 11 Aug 2025 09:25:09 -0300
From: Enzo Matsumiya <ematsumiya@...e.de>
To: David Howells <dhowells@...hat.com>
Cc: Steve French <sfrench@...ba.org>, Paulo Alcantara <pc@...guebit.org>, 
	Shyam Prasad N <sprasad@...rosoft.com>, Tom Talpey <tom@...pey.com>, 
	Wang Zhaolong <wangzhaolong@...weicloud.com>, Stefan Metzmacher <metze@...ba.org>, 
	Mina Almasry <almasrymina@...gle.com>, linux-cifs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/31] netfs: [WIP] Allow the use of MSG_SPLICE_PAGES
 and use netmem allocator

On 08/11, David Howells wrote:
>Hi Enzo,
>
>I now have encryption, compression and encryption+compression all working :-)
>
>I've pushed my patches here:
>
>	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=cifs-experimental
>
>It should work up to "cifs: Don't use corking".

Great! I'll try it out later.

>Btw, is is_compressible() actually worth doing?  It seems to copy a lot of
>data (up to 4M) to an extra buffer and then do various analyses on it,
>including doing a sort.

Compression, as a whole, is actually only worth doing if one is paying
more for network traffic than computing.  is_compressible() tries to
balance that to avoid a "compress/fail/send original" cycle, as it takes
0-4ms on a 4M payload (on my machine) whereas, without it, a failing
cycle can take up to 40ms.

>I need to extract a fix for collect_sample(), which I can do tomorrow, but it
>should look something like:
>
>/*
> * Collect some 2K samples with 2K gaps between.
> */
>static int collect_sample(const struct iov_iter *source, ssize_t max, u8 *sample)
>{
>	struct iov_iter iter = *source;
>	size_t s = 0;
>
>	while (iov_iter_count(&iter) >= SZ_2K) {
>		size_t part = umin(umin(iov_iter_count(&iter), SZ_2K), max);
>		size_t n;
>
>		n = copy_from_iter(sample + s, part, &iter);
>		if (n != part)
>			return -EFAULT;
>
>		s += n;
>		max -= n;
>
>		if (iov_iter_count(&iter) < PAGE_SIZE - SZ_2K)
>			break;
>
>		iov_iter_advance(&iter, SZ_2K);
>	}
>
>	return s;
>}
>
>What's currently upstream won't work and may crash because it assumes that
>ITER_XARRAY is in use - which should now never be true.

Yes, compression was merged when that was the only case.

>Also, there's a bug in wireshark's LZ77 decoder.  See attached patch.

Good catch :)
There are several, actually... if you vary the compression parameters
defined (min len, min/max dist, hash log) within acceptable limits,
you'll notice that, even though wireshark might show some as malformed
packets, Windows is able to decode them just fine.

I really need to reserve some time to work on this again :(


Cheers,

Enzo